Single-molecule motor sits on a single-atom ball bearing

Can be run forward or in reverse, depending on where electrons are injected.

The base of the device holds a Ru atom, and the five-armed device can rotate on top of it.

Image from Perera et. al., Nature Nanotechnology

For some time now, researchers have been managing to craft ever-smaller devices, though they're approaching the problem from two directions. Some researchers are etching small features into chips to carve out nanoscale versions of familiar devices. But others are taking advantage of our ability to synthesize and interact with individual molecules to create systems that are only a few dozen atoms across. And, in many cases, these single-molecule devices look disturbingly like their full-scale counterparts.

When last we left single-molecule motors, they were four wheeling across a sheet of copper, powered by electrons fed in by an atomic force microscope. In the latest iteration, researchers have managed to create a reversible rotor that sits atop a ball bearing—but in this case, the bearing is a single ruthenium atom.

Again, the tricky part comes in building the molecules required. The base of the system involves a boron atoms that coordinates three ringed structures that are chemically similar to the bases of DNA. Nitrogens at a corner of these ringed structures coordinate the ruthenium atom, placing it at the peak of a three-sided pyramid. (This compound has the succinct name [n5-1-(4- tolyl )-2,3,4,5-tetra(4-ferrocenylphenyl) cyclopentadienyl hydrotris [6-((ethylsulphanyl)methyl)indazol-1-yl] borate ruthenium(II)], which should provide some sense of its complexity.)

The authors cleaned off a gold surface, and then dropped the pyramids onto it. With those in place, they layered on something that looks a bit like a five-armed windmill. A five-atom ring sits at the center, with further rings extending out at each corner to form the blades of the windmill. For four of the five arms, the authors placed a complex of rings surrounding an iron atom. The fifth arm was left blank so that they could actually tell when something shifted. (If all arms were identical, it would be impossible to tell different configurations apart.)

It was possible to arrange this molecule so that the central five-carbon ring sat directly on top of the ruthenium atom. That let the ruthenium act like a ball bearing, allowing the molecule sitting atop it to rotate, spinning like a windmill tilted on to its back, with its blades oriented horizontally.

To actually get it to rotate, a scanning-tunneling microscope was again used to inject electrons into the system. The added charges allowed the rings to overcome interactions with the base, and click one position forward (with five positions, corresponding to the five arms, providing a complete rotation). But the authors could also control the direction of rotation. Pump the electrons into one of the iron-containing arms, and the motor would turn in one direction; put them into the single truncated arm, and it would turn in the opposite one.

It's a pretty neat demonstration of fine-scale control on the molecular level, and it adds to our knowledge of the sorts of single-molecule devices we can build. For the moment, however, even though these things are single molecules, we're still dependent upon some pretty serious external hardware (the scanning-tunneling microscope) to power them.

It's a pretty neat demonstration of fine-scale control on the molecular level, and it adds to our knowledge of the sorts of single-molecule devices we can build. For the moment, however, even though these things are single molecules, we're still dependent upon some pretty serious external hardware (the scanning-tunneling microscope) to power them.

Is there some serious application for this , at least at sci-fi level? It occurs to me something like tissue repair,...

These things seem to be stretching the concept of "motor" a bit, since you can't drive anything with it. About all it does is "rotate".

If that's the criterion, I've got a much smaller "motor" I've come up with. You take a proton, and you put an electron rotating around it... and it just goes! Perpetual motion! You can control the speed by hitting it with a photon and boosting it to a higher orbital, and you can slow it down by letting it emit a photon.

I await my inevitable Nobel.

Of course, to be a real motor it must produce useful work. You rather see this as an exercise of technical and theoretical scope. Your example does not.

It's a pretty neat demonstration of fine-scale control on the molecular level, and it adds to our knowledge of the sorts of single-molecule devices we can build. For the moment, however, even though these things are single molecules, we're still dependent upon some pretty serious external hardware (the scanning-tunneling microscope) to power them.

Is there some serious application for this , at least at sci-fi level? It occurs to me something like tissue repair,...

It's a pretty neat demonstration of fine-scale control on the molecular level, and it adds to our knowledge of the sorts of single-molecule devices we can build. For the moment, however, even though these things are single molecules, we're still dependent upon some pretty serious external hardware (the scanning-tunneling microscope) to power them.

Is there some serious application for this , at least at sci-fi level? It occurs to me something like tissue repair,...

A couple thousand years ago, we were able to create machinery like this, that were the size of a house and required immense energy to move. Today we have machinery so small and efficient it can take the tiny energy in the fumes off refined oil, and propel a few tons of cargo down a road at high speed.

Who knows what will be possible in another thousand years; but we won't get there without doing research like this and spreading the results worldwide.

Seems boron is used to save complexity of the piedestal molecule, else carbon would be the usual choice. And they use a rather asymmetric rotor molecule instead of, say, a fluorescent molecule additive to see motion, since it somehow allow them to decide motor direction.

But if they take all that care to snazzy up the motor, who do they still supply it with electrons by probe injection?

The papers are paywalled, so it is not doable to see how much charge is needed to rotate the carbon rings. It is confusing to me why they draw what looks to be aromatic resonance ring molecules as alternating single/double bond molecules, but at least the axis ring molecule is clearly aromatic (drawn with inner ring). So presumably you need much less induction currents (or whatever) to shift the delocalized resonances as opposed to the pi-bond of double bonds.

Isn't it enough to use some GFP like electron donors and excite them with light? (Since it seems the acceptor side is ground.)

You may need a lot of donors depending on efficiency, and then have to figure out how to replenish them. Here something like the probe comes in handy as remote electron spray gun. And GFP itself may not work outside of solution of course, I wouldn't know. But perhaps you can get the whole motor a lot more independent.

These things seem to be stretching the concept of "motor" a bit, since you can't drive anything with it. About all it does is "rotate".

If that's the criterion, I've got a much smaller "motor" I've come up with. You take a proton, and you put an electron rotating around it... and it just goes! Perpetual motion! You can control the speed by hitting it with a photon and boosting it to a higher orbital, and you can slow it down by letting it emit a photon.

I await my inevitable Nobel.

Of course, to be a real motor it must produce useful work. You rather see this as an exercise of technical and theoretical scope. Your example does not.

Uh, my first sentence points out that the subject of this article doesn't do any work either. That's my *point*.

It does internal mechanical work, as opposed to electron clouds of single* atoms. It should be easy to hook this motor up to move stuff.

* Feasibly you can make electron clouds of molecules do such work however, as you can move stuff around. Indeed, that is what this motor relies on.

Or maybe you can use somehow anchored atoms as magnetic coupled engines, since they have spin and resulting magnetic moments that you could use to push stuff around by delivering external energy. But I suspect we are describing molecules again (as making an anchor).

It is awesome how quantum mechanics works around the thermodynamics of (not) doing "work", so atoms can be stable. Else electrons would lose energy and spiral into the nucleus, purely from EM effects.

Speaking from a layman. Great, congrats to the scientists, another science breakthrough. Is it not? My question would be when dealing with objects as tiny as a size of a few single atom across, do we really need a ball bearing on them? What I understand the purpose of a ball bearing is it's necessary to have in heavy machinary, it is to reduce friction to cut down on heat to prevent parts from wear and tear. Is this a big concern to have a ball bearing on size of a few atoms? How is the heat generated from this tiny piece of molecule and the wear and tear part is..? On the other hand, it seems to me, I'm more interesting on this scanning-tunneling microscope. Sounds like it should be useful on reparing damage nerves. It may be something as tiny as brain nerves? Well as we already have something similar for repairing damage human skin tissues or use it to seal the wounds with laser beam. But this microscope may be something else.

Quote:

I wonder if THAT might be a sufficiently secure password for my Hotmail account...

Wonder where Hotmail would truncate the "password" to fit within their 14(or16?) byte length? Would that even be a stable chemical? (I'm not a chemist or a chem-engineer.) Of course, you've just added the CRC to the list of dictionaries used to "attack" Windows computers.

The system modeler in me really wants to know what the torque curve looks like. The next step is to use one in a lab for control theorists. Can an inverted pendulum problem exist at the molecular scale?

It appears not dissimilar from ATP Synthase, which is the last enzyme in oxidative phosphorylation. After pumping protons across a mitochondrial cell membrane to create a concentration gradient, their flow through ATP Synthase causes phosphate ions to be crammed onto ADP, forming ATP, which is sort of like compressing a spring-loaded mechanism to be fired off for energy at a later time. ATP Synthase spins as the protons flow through it, like a nanoscale generator.

It does internal mechanical work, as opposed to electron clouds of single* atoms. It should be easy to hook this motor up to move stuff.

It's not clear what internal mechanical work it is doing by spinning a rotor in place. Work has a very specific definition, and this doesn't fit it. And I'm sure it would actually be devilishly hard to hook it up to anything. You know what you get if you have one molecule spinning near another molecule that isn't spinning? Nothing. You'd have to find a way to couple this with a nearby molecule in a way that doesn't destroy the delicate balance that makes this molecule work.

This is really cool lab work and I applaud this as great basic, blue-sky science. But extending the work will continue to be hard. And there really are no practical applications for a device like this. Spinning a small rotor on that scale basically does absolutely nothing.

It does internal mechanical work, as opposed to electron clouds of single* atoms. It should be easy to hook this motor up to move stuff.

It's not clear what internal mechanical work it is doing by spinning a rotor in place. Work has a very specific definition, and this doesn't fit it. And I'm sure it would actually be devilishly hard to hook it up to anything. You know what you get if you have one molecule spinning near another molecule that isn't spinning? Nothing. You'd have to find a way to couple this with a nearby molecule in a way that doesn't destroy the delicate balance that makes this molecule work.

This is really cool lab work and I applaud this as great basic, blue-sky science. But extending the work will continue to be hard. And there really are no practical applications for a device like this. Spinning a small rotor on that scale basically does absolutely nothing.

Well, it's already an unbalanced system since one of the arms was lacking an atom. That tells me there's enough resiliency to having an off-balance load to add and then remove an atom on the leaves as they pass around. What good would that do? How about fabrication on an atom-by-atom basis? Use some sort of logic (either external or programmed into the surrounding chemistry a la RNA/DNA) to select what order atoms should be deposited in a grid, array, etc. Assemblies like this could be used to pass the atoms along conveyor-belt style. The only challenge left is sending the electrons to the proper arms. This is not a trivial problem but it's obviously high on everyone's minds that works in this field. If, in fact, the ATP reaction of photosynthesis could be modified to generate such electrons then it's simply a matter of coupling such a device to a silicon-based substrate with LED at the appropriate junctions.

It's a stepper motor so there is excess torque already available.

This is sort of like figuring out FET transistors. In ones and two's they're a curiosity. In dozens you can do some proof-of-concept logic. By the time you can address and control hundreds of them you've got an early mricroprocessor and all the infrastructure to move to VLSI design. Now you're at a modern CPU.

Obviously the ability to power the device in a controlled fashion is a critical step. But at least we know it's a step worth taking once you know what you have planned for it.